The present invention relates to an image forming apparatus, and in particular, to an image forming apparatus wherein an image is fixed on a recording material through heat fixing.
In an electrophotographic image forming apparatus or the like, there is commonly used a heat fixing device wherein an unfixed toner image is heated so that toner is heated and melted to be fixed on a recording material.
In the case of heat fixing, there is required a waiting time called a warm-up time that covers a period of time from the moment when a heater of the fixing device is turned on to the moment when a temperature of the heater arrives at a level for fixing, and there have been made various suggestions for shortening the warm-up time, because users wish to have a shorter waiting time.
For example, Patent Document 1 discloses a technology to lower the temperature capable of fixing to shorten the warm-up time, by lowering a speed for conveying a recording material in the fixing device, that is, a fixing speed.
In the device of the Patent Document 1, the amount of heat reduced by the reduction of the fixing temperature is compensated with the reduction of the fixing speed, resulting in advantages that a problem of insufficient fixing is mitigated considerably, and an effect of reduction of the warm-up time is obtained sufficiently.
However, the device of the Patent Document 1 has the following problems.
When the fixing speed, namely, the conveyance speed for a recording material in the fixing device is lowered in conformity to the method of the Patent Document 1, copying time is made longer by the reduction of the fixing speed and the time required for copying grows longer, and efficiency is lowered on the contrary, when the number of copies is large, although the time required for copying is shortened by the reduction of the warm-up time when the number of copies is small.
The foregoing will be described as follows, referring to FIG. 1.
When ten (10) copies are made under the condition of warm-up (hereinafter referred to as WP) reduction, the time required for copying is WP1+10CP1 as shown in FIG. 1(a).
Under the condition of regular WP, the time required for copying is WP2+10CP2 as shown in FIG. 1(b), and the expression (WP1+10CP1)<(WP2+10CP2) holds because WP1 is shorter than WP2, thus, the time required for copying is shortened by shortening of WP.
In the case of 100 copies, however, the expression of (WP1+100CP1)(WP2 +100CP2) holds, as shown in FIGS. 1(c) and 1(d), because 100CP1 becomes extremely longer than 100CP2, and the time required for copying becomes longer extremely, in spite of shortening of WP.
As stated above, any of the conventional technology for shortening a warm-up time is not sufficient, and it has a problem, in particular, that the reduction of the warm-up time desired by a user cannot be attained.
Though a technology to shorten the warm-up time has been developed by an improvement of a fixing device, on the other hand, warm-up time is needed for those other than a specific image forming apparatus such as a small-sized apparatus, and an image forming apparatus having a warm-up time satisfied by a user has not been realized, although a fixing device requiring no warm-up time has been developed in the field of a small-sized apparatus or a low-speed apparatus.
[Patent Document 1] Japanese Patent Open to Public Inspection No. 2003-66765